This replaces the most usages of MonadPlus, which is not appropriate for
the resolvers, since a resolver is unambiguously chosen by the name (no
need for 'mplus'), and the resolvers are often doing IO.
Now the errors in the resolvers can be handled and 3 tests throwing
errors pass now. Another test fail but it requires distinguisching
nullable and non-nullable values.
One AST is meant to be a target parser and tries to adhere as much as possible
to the spec. The other is a simplified version of that AST meant for execution.
Also newtypes have been replaced by type synonyms and NonEmpty lists are being
used where it makes sense.
Aside of making the definition of Schemas easier, it takes care of
issues like nested aliases which previously wasn't possible. The naming
of the DSL functions is still provisional.
The `Schema` has been overhauled to make `Output` monomorphic.
Traversing the `GraphQL` document is handled implicitly while defining
the `Schema`.
The 4th end-to-end test from `graphql-js` has been ported.
`Character` is now a synonym of the sum type of `Droid` and `Human`.
For now I don't see the need to implement GraphQL Schema interfaces with
type classes or lens. Plain Haskell ADTs should be good enough.
The first end-to-end test taken from `graphql-js` passes but this still
needs to be extended to support more general cases.
- `Data.GraphQL.Schema` has been heavily modified to support the
execution model. More drastic changes are expected in this module.
- When defining a `Schema` ordinary functions taking fields as input are
being used instead of maps. This makes the implementation of `execute`
easier, and, arguably, makes `Schema` definitions more *Haskellish*.
- Drop explicit `unordered-containers` dependency. `Aeson.Value`s and
field functions should be good enough for now.
